One method of determining the relative proportions of the isotopes of a chemical element present in a material is to subject the material to analysis by mass spectrometry. The type of mass spectrometer usually employed consists of an ion source for generating a beam of ions which are characteristic of the element(s) in the sample for which the isotopic analysis is required, a mass selector, which is usually a magnetic field at right angles to the direction of motion of the ions arranged to deflect the ion beam so that ions of different mass to charge ratios follow different trajectories, and an ion detector which produces an electrical signal related to the number of ions falling on it. A narrow slit (termed the collector slit) is positioned in front of the ion detector so that ions of only one particular mass to charge ratio can fall on the ion detector. As with all mass spectrometers, the path along which the ions travel must be maintained at a high vacuum. By variation of the strength of the magnetic field which deflects the ion beam, ions of different mass to charge ratio can be made to pass through the collector slit, and the relative intensities of different beams can be measured thus allowing the determination of the relative proportions of the isotopes present in the sample. Such mass spectrometers are well known and need not be described further.
When a mass spectrometer of this type is used to determine the ratio of the intensities of two ion beams corresponding to the isotopes of an element present in a sample, the magnetic field strength has first to be adjusted to allow ions of the first mass to charge ratio through the collector slit, and then readjusted to allow ions of the second mass to charge ratio to be determined. The accuracy of the ratio measurement will therefore be reduced if the total number of ions produced by the source should vary for any reason during the time that the measurements are being made. In practice, variations of this kind cannot be eliminated. It is known that this problem can be circumvented by the provision of two or more collector slits, each equipped with an ion detector, in place of the single collector slit and detector. These slits and detectors are positioned so that the first slit receives ions of the first desired mass to charge ratio, whilst simultaneously the second slit receives ions of the second desired mass to charge ratio, and so on. In this way the ion beam intensities corresponding to a number of mass to charge ratios can be measured simultaneously, and their values compared directly, so that the ratio between them can be determined accurately even if the absolute value of the intensity of all the beams changes for any reason. Another advantage of this method is that the time needed for the determination of a ratio is reduced in comparison with the method using only one collector slit and detector, because the measurements are made simultaneously. This means that a ratio can be accurately determined with a smaller quantity of sample than is necessary for the single collector system.
It will be appreciated that in separating ion beams of different mass to charge ratio, the magnetic field focusses the ion beam of a particular mass to charge ratio on to the collector slit, and that the position of the ion source and the collector slit, and the shape and size of the magnet poles, have to be selected to obtain the optimum focussing properties. If this is not done, the ion beams corresponding to only slightly different mass to charge ratios will overlap and their intensities cannot be accurately determined. The theory which may be used to calculate the geometrical arrangement of mass spectrometers to obtain the best results is well known and need not be described. This theory shows, and its predictions are confirmed in practice, that for a particular magnetic field strength, ion beams of different mass to charge ratios from the one focussed on the collector slit are in fact focussed at different points, and that these points lie on a plane which passes through the collector slit and is generally inclined at an angle to the ion optical axis of the magnet. In most cases, this angle is considerably smaller than 90.degree.. In order to construct a multiple collector mass spectrometer of the type described it is therefore necessary to arrange the collector slits along this focal plane at the focal point of each of the ion beams which are to be determined, if resolution between the ion beams is to be maintained. Mass spectrometers constructed in this way are known, but suffer from two defects which detract from their performance.
The first defect is that the sensitivity and accuracy are lessened by the presence of a background signal on certain collectors in the presence of an intense ion beam falling on an adjacent collector, even when no ion beam is falling directly on the first collector. In general, collectors mounted behind the collector receiving the intense beam are the worst affected and the problem is attributed to off axis ions in the intense beam striking part of the collector assembly for that beam at a shallow angle and being deflected into an adjacent collector, especially one mounted behind the collector receiving the intense beam. This is a particular problem when measuring isotopic ratios because it is often necessary to measure the ratio of intensities of two beams differing in mass to charge ratio by only 1 or 2 daltons, when one beam is significantly less than 1% of the main beam. To obtain satisfactory performance it is therefore necessary to fit screens between the collector assemblies, but it is very difficult, if not impossible, to make these completely effective when the collectors are staggered along the focal plane, at a shallow angle to the direction of motion of the ions.
The second defect of the method is that if it is desired to change the mass/charge ratios for which the collectors have been positioned to receive, so that a different isotopic ratio can be determined, it is necessary to completely dismantle the collector assembly of the mass spectrometer and to make difficult mechanical modifications. In fact, if the changes required are large, an entirely new collector assembly may have to be manufactured. This difficulty arises because the spacing of the collectors for ion beams of different mass to charge ratios is dependent on the actual value of the mass/charge ratio as well as the difference between them. Further, because the magnets used in practice do not behave exactly in the manner predicted by theoretical treatments, it is not possible to calculate accurately the spacing needed between collectors for a particular set of mass to charge ratios. During the initial setting up procedure it is therefore necessary to make small adjustments to the positions of the collectors in order to ensure that they receive only the correct ion beam. Because each adjustment requires admitting air into the spectrometer and at least partial dismantling of the collector assembly, this procedure is very time consuming. Clearly, there would be considerable advantage from making the positions of the collectors adjustable from outside the vacuum system, but it is difficult to construct a mechanism which will move them along the focal plane of the magnet, because of its shallow inclination to the ion optical axis. Mechanisms which move the collectors at right angles to the optical axis are known, but are unsatisfactory because in moving the collector they also move it off the focal plane, thus reducing the resolution of the instrument. Consequently, these mechanisms can only be used for a very small range of adjustment, which is not sufficient to allow the instrument to be tuned to monitor different isotopic ratios.
It is the object of the present invention to provide a multicollector mass spectrometer which overcomes these difficulties, and which consequently has higher sensitivity and accuracy, and is easier to use and adjust, than previously known types.